1. Technical Field
The present invention generally relates to semiconductor structures and the fabrication thereof. More particularly, the present invention relates to a semiconductor structure and method of making same having a semiconductor layer on a substrate, the layer having a strain relaxation degree above about 80% and a threading dislocation density of less than about 100/cm2.
2. Background Information
Strain-relaxed semiconductor material on a semiconductor substrate, such as silicon germanium on a silicon substrate, has numerous potential applications for electrical and opto-electrical devices. For practical applications, it is desirable that the layer have a high degree of strain relaxation, a low threading dislocation density, and a smooth surface. In addition, minimizing the layer thickness is also desirable, because as the layer thickness increases, production costs rise and significant technological issues occur, such as poor thermal conductivity associated with the material. These desired characteristics are often self-contradictory according to experimental results and theoretical modeling. For example, both experimental results and theoretical models indicate that the strain relaxation degree for SiGe on Si substrate depends on the SiGe layer thickness—the thicker the layer, the higher the strain relaxation degree. A high degree of strain relaxation (on the order of 90%) is expected only for very thick films, but is not practical in terms of cost. Similarly, the threading dislocation density (TDD) is shown as a function of the SiGe layer thickness, whereby TDD decreases with increasing SiGe layer thickness. As a result, it is a challenge to manufacture thin SiGe on Si substrate with a high degree of strain relaxation and low TDD for device applications.
A need therefore exists for the cost-effective manufacture of a strain-relaxed semiconductor layer with a high strain relaxation degree, a low TDD, and a reduced layer thickness that is viable for volume production.